Following part design and material selection the next step in plastic injection molding is getting the tool designed and made. One of the earliest decisions to make is whether to have a cold runner or hot runner system.

Cold Runner

When a mold is designed for a cold runner system, you have a channel formed between the two halves of a mold, allowing the plastic to move from the injection molding machine nozzle to the cavities. When the mold opens to eject the newly formed parts, the material in the runner system is also ejected, resulting in scrap material.

Hot Runner

This system is an assembly of heated components that inject molten plastic into the cavities of the mold. A hot runner system typically includes a heated manifold and a number of heated nozzles. When a mold with a hot runner system opens, only the part ejects as material in the runner system is kept molten and will fill into the part cavity during the next cycle.

So which do you choose?

Benefits of a Hot Runner System:

Eliminate the runner thus eliminating expensive scrap (and potential regrind issues) and you have less handling of materials.

Lower the cycle time since you are not waiting for the cold runner to cool during the cycle. Removing the runner also improves the injection screw recovery and injection time since due to the smaller shot size

Design flexibility because you can locate the gate at many points on the part

Downsides of a Hot Runner System:

Very expensive to design and build

Maintenance of the mold requires higher level of expertise

Complicated design

Benefits of a Cold Runner System:

Less expensive to manufacture

Lower maintenance costs

Easier to use for a wide variety of polymers

Downsides of a Cold Runner System:

Scrap waste through runner system and handling of materials

Potentially incorporating regrind into the system

When you are making the decision to choose between the a cold or hot runner system it is important to have a thorough understanding of your part, the material, and the estimated annual units. While hot runner systems are expensive to make, the scrap material waste and extended cycle time can offset the tooling savings.